The invention relates to the preparation of chemical pulp and xylose, and particularly to recovering xylose from pulp, such as sulphate pulp, prepared by alkaline or neutral cooking, and simultaneously achieving the desired characteristics for the pulp. The method of the invention utilizes a direct acid hydrolysis of the pulp, resulting in a good xylose yield. At the same time, the obtained pulp is usable as paper pulp or dissolving pulp.
In a plurality of plants, the main portion of hemicellulose is xylan, which can be hydrolyzed to xylose. The foremost starting material for xylan is hemicellulose from hardwood, particularly birch, mainly composed of xylan.
For example birch sulphate pulp typically contains about 15 to 25% xylan, which is usable as a raw material of xylose. When xylose is prepared from pulp, the problem involved has been to achieve sufficient xylose yields and to simultaneously achieve acceptable characteristics for the pulp.
Finnish Patent 55516 (Kemi Oy) discloses a method of preparing pure xylan, suitable for the raw material of xylose and xylitol, in particular. The method uses bleached or unbleached hardwood cellulose as the raw material. The cellulose is treated with an alkali solution, whereby the hemicelluloses are dissolved. The alkali solution containing hemicellulose is pressed and filtered from pulp. The dissolved hemicellulose is precipitated by the addition of carbon dioxide to the solution, whereby the xylan precipitates. In the method, most of the xylan in the pulp, the xylan being in principle usable as the raw material of xylose, is, however, not utilized. Moreover, the method uses much alkali.
Several methods are also known in which enzymatic hydrolysis is used to separate hemicellulose components from the pulp. For example Paice, M. G. and Jurasek, L., Removing Hemicellulose from Pulps by Specific Enzymic Hydrolysis, J. Wood Chem. and Tech., 4 (2), 187 to 198, 1984, describes a method of separating hemicellulose from aspen pulp by xylanase treatment. The most important hemicellulose products thus obtained were xylan and xylobiose. However, the enzyme dose is uneconomically large.
WO 98/56958 (Xyrofin Oy) discloses a method of preparing xylose by first extracting xylan from a cellulose pulp or its alkali solution with an aqueous solution of a xylanase enzyme and by then using acid to hydrolyze the xylan in the solution to xylose. However, the acid hydrolysis is not performed directly on the chemical pulp, and therefore all the xylan in the pulp cannot be utilized.
Acid hydrolysis is also applied as pre-hydrolysis in the separation of xylose from wood material. In this case, the acid hydrolysis is performed directly on wood chips before the pulp is prepared. One such method is disclosed in Guangyu, Yao et al., Production of Pulp and Recovery of Xylose from Hardwood. II. The Optimal Process Conditions for Prehydrolysis of Eucalyptu Citriodora Chips with Dilute Sulphuric Acid and Sulfate Pulping, Journal of Nanking Technological College of Forest Products, No. 4 (1988), p. 32. The publication relates to a method of preparing sulphate pulp with simultaneous recovery of xylose. As raw material is used Eucalyptu Citriodora chips, on which prehydrolysis is carried out with dilute sulphuric acid. A xylose-containing solution and prehydrolyzed wood chips are obtained. Sulphate pulp is then prepared from the prehydrolyzed wood chips. The pulp is said to be suitable for the preparation of viscose, for example.
U.S. Pat. No. 4,008,285 (Melaja, A. J. and Hxc3xa4mxc3xa4lxc3xa4inen, L.) discloses a method of recovering xylitol from a xylan-containing raw material, which may be for example wood material, such as birch chips. The birch chips are first hydrolyzed with for example acid, the hydrolysate is purified and the purified hydrolysate is subjected to chromatographic fractionation to provide a solution containing a high level of xylose. However, the pulp is not recovered in this method.
It is therefore an object of the invention to provide a method of recovering xylose from pulp prepared by alkaline or neutral cooking with a sufficient xylose yield, and, simultaneously, preparing paper pulp or dissolving pulp so as to obtain acceptable characteristics for the pulp. The objects of the invention are achieved by a method, which is characterized in what is disclosed in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
In accordance with the present invention, it has now been surprisingly found that high-quality paper pulp and dissolving pulp can be prepared by first subjecting the pulp to alkaline or neutral cooking and then, as post hydrolysis, to acid hydrolysis in order to recover the xylose. In the method of the invention, simultaneous extraction and hydrolysis of xylan are achieved, and extensive use of alkali in the extraction of xylan can be totally avoided.
In the context of the present invention, the expression xe2x80x98sufficient xylose yieldxe2x80x99 refers to a xylose yield of at least 5% (50 g xylose/1,000 g pulp), preferably at least 10% (100 g xylose/1,000 g pulp), calculated on the dry substance of the pulp.
In the context of the present invention, the expression xe2x80x98acceptable pulp characteristicsxe2x80x99 means that the viscosity of the acid-treated pulp remains sufficient for paper pulp or dissolving pulp. Typically, the viscosity of paper pulp or dissolving pulp should be at least 300 ml/g, preferably at least 450 ml/g, and most preferably at least 600 ml/g. The acceptable viscosity values depend on the final purpose of use of the pulp. If the pulp is used for the preparation of paper whose strength characteristics have to be good, a higher viscosity is required for the pulp, typically at least 600 ml/g. Pulp having a lower viscosity is feasible particularly when acid-treated pulp obtained by the method of the invention is used in a mixture with non-acid-treated pulp in the production of paper, for example.
The expression xe2x80x98post hydrolysis is performed directly on the pulpxe2x80x99 means that the acid treatment for hydrolyzing xylan to xylose is performed on the pulp itself, not for example on a xylan solution extracted from the pulp (such as in the method of WO 98/56958, for example). In this case, xylan is hydrolyzed into xylose in connection with the acid treatment of the pulp.
The present invention relates to a method of preparing paper pulp or dissolving pulp and a xylose solution by the use of alkaline or neutral cooking and post hydrolysis of the pulp. The method is characterized in that the post hydrolysis is performed directly on the pulp by the use of an acid.
In the method of the invention, the pulp is typically post-hydrolyzed with an acid until the xylose yield is at least 5%, preferably at least 10%, while the viscosity of the pulp remains at a value of at least 300 ml/g, preferably 450 ml/g.
When the hydrolysis of xylan is performed directly on the pulp in connection with the acid treatment of the pulp without pre-treatment steps, the xylan contained by the pulp can be utilized as completely as possible. In this case the xylan can be hydrolyzed as efficiently as possible into xylose, and the xylose yields can be optimized.
In the present invention, alkali cooking refers to a sulphate process, soda process, soda/anthraquinone process and alkali sulphite process, and neutral cooking to a neutral sulphite process. Post hydrolysis is thus typically performed on pulp prepared by alkaline or neutral cooking, typically the sulphate process, the soda process, the soda/anthraquinone process, the alkali sulphite process or the neutral sulphite process. Sulphate pulp is the most preferably used.
In the cooking, hardwood or herbaceous plants can be used as the raw material. Examples of usable hardwood include birch, aspen, alder, oak, poplar, beech, gum tree and acacia tree. Especially important are birch and beech, for example. Examples of herbaceous plants include reed, reed canary grass, bagasse, bamboo and straw, such as corn straw.
An especially preferable raw material is birch, whereby acid hydrolysis is performed on birch sulphate pulp.
In the cooking, the pulp is cooked to the desired kappa number, which is typically in the range between 20 and 40. After the cooking, the pulp may be further subjected to oxygen delignification (to a kappa number of about 10, for example) and bleaching (typically to a kappa number of about 0).
The acid hydrolysis can be performed immediately after the cooking, oxygen delignification or bleaching (ECF bleaching, for example).
In order to make the xylan hydrolyze as efficiently as possible to xylose, the pulps can be subjected to liquor exchange before the acid treatment, whereby the solution affecting the pulp is as close as possible to the adjusted concentration. In liquor exchange, the aqueous solution contained by the pulp is replaced by the acid to be used, for example by concentrated formic acid. The liquor exchange can also be carried out by evaporating the water contained by the pulp and replacing it with the acid to be used in the acid treatment.
In the acid treatment, xylose is preferably recovered in monomer form. However, xylose can also be recovered in oligomer form.
The acid treatment can be carried out with a mineral acid or an organic acid.
The acid treatment is preferably carried out with formic acid. The concentration of the formic acid solution is typically in the range between 50 and 100%, preferably between 75 and 90%. The treatment temperature is typically between 90 and 130xc2x0 C., preferably between 100 and 120xc2x0 C. The duration of the formic acid treatment is typically from 15 min to 4 h, preferably from 20 min to 1.5 h.
The acid treatment can also be carried out with for example a bisulphite solution, which is usually readily available in sulphate pulp processes. In the present invention, a bisulphite solution refers to a partly neutralized aqueous solution of sulphur dioxide (SO2) containing bisulphite ions. The SO2 content of the bisulphite solution is typically in the range of about 1 to 5%, preferably about 3%, of which the amount of bound SO2 is typically about 10%. When a bisulphite solution is used, the acid treatment temperature is typically about 110 to 150xc2x0 C., preferably about 125 to 145xc2x0 C. The duration of the bisulphite treatment is typically about 1 to 3 h.
Other usable acids include sulphuric acid, sulphurous acid and hydrochloric acid.
The treatment with formic acid is preferably carried out after bleaching (for example ECF bleaching), but it also can be carried out after oxygen delignification, or even immediately after cooking, on unbleached pulp.
The treatment with bisulphite solution is preferably carried out before oxygen delignification/bleaching, but can also be carried out after oxygen delignification or bleaching, for example ECF bleaching.
After the acid treatment, the pulp is typically washed to recover the xylose as completely as possible from the acid-treated pulp.
After the acid treatment, the obtained xylose solution and the pulp are separated, typically by filtration. The acid used, for example formic acid, is then separated from the xylose solution, typically by distillation. The separated acid is recycled and reused in hydrolysis.
The obtained xylose solution is usable for the preparation of xylose. From the xylose, xylitol can be further prepared by catalytic reduction, for example.
The xylose is usable as such, for example as a flavour and an aroma intensifier. The xylitol is usable as a special sweetener, for example.
The paper pulp or dissolving pulp obtained from the acid treatment is recovered. The pulp thus obtained is usable, when bleached, for the preparation of paper and viscose, either as such or in combination with non-acid-treated pulp.
The invention also relates to a xylose solution and to a pulp product obtained by the method and to xylose obtained from the xylose solution. The invention also relates to the use of the xylose solution thus obtained for the preparation of xylose and xylitol, and to the use of the obtained paper pulp and dissolving pulp for the preparation of paper or viscose. The invention also relates to the use of the xylose thus obtained for the preparation of flavours and/or aroma intensifiers.
The following detailed examples illustrate the present invention.
In the examples, the kappa number, viscosity, pulp yield (on dry substance of the pulp) and brightness were determined by the following methods:
Kappa number SCAN-C 1:77
Viscosity SCAN-CM 15:88
Pulp yield (on dry substance of the pulp) SCAN-C 3:78
Brightness SCAN-C 11:75.
The xylose yields (as % of dry substance of the pulp) were calculated by means of the xylose content analyzed from the cooking liquor by HPLC and by the consistency of the cooking.